The present invention generally relates to a copier, facsimile apparatus or similar image forming apparatus of the type including a fixing unit and, more particularly, to a device for displaying a waiting time necessary for the warm-up of a heating element of such an image forming apparatus.
There has been known a copier, facsimile apparatus or similar image forming apparatus of the type including a fixing unit, a temperature sensor responsive to the temperature of the fixing unit, and a display responsive to an output of the temperature sensor for determining and displaying a waiting time which is necessary for warming up the apparatus. It is a common practice with this type of image forming apparatus to sense the temperature of the surface of a fixing element in the form of a fixing roller by using a thermistor. One approach available for displaying the sensed temperature is transforming an output of the thermistor into a voltage and sequentially turning on a plurality of light emitting diodes in respone to the variation of the voltage. Another approach is displaying the sensed temperature on a display which is adapted to show the number of copiers.
In practice, however, the waiting time necessary for the warm-up of a fixing unit is dependent upon the elevation rate of temperature which is in turn dependent upon the input voltage, ambient temperature, etc. Especially, since a thermistor associated with a fixing roller is designed to operate at temperatures of 180.degree. C. to 190.degree. C., it is incapable of sensing temperatures close to room temperature with accuracy. Heretofore, no consideration has been given to the differences of input voltage and the variations of ambient temperature in determining a waiting time based on the temperature elevation rate, resulting in inaccurate detection of waiting time. This problem may be eliminated by sensing the temperature of a fixing roller, determining a waiting time by using a predetermined temperature elevation rate, and displaying such a waiting time, as disclosed in Japanese Laid-Open Patent publication No. 56-164376 by way of example. The present invention constitutes an improvement over the implementation disclosed in this Laid-Open Patent Publication.